Many have proposed that the goal of visualization is insight. However, few rigorous definitions of insight exist in the visualization community, and none have been commonly accepted. We propose that two types of insight are relevant to the visualization community. We examine the definition of insight cognitive scientists use (spontaneous insight) and compare it with the definitions the visualization community uses (knowledge-building insight). The two, although distinct, are related and contribute toward each other. Only by understanding how the two types differ and interact with each other can the visualization community accurately measure insight and determine the effectiveness of visualizations.